1. Field of the Invention
The present invention relates to a novel polyimide having a rigid three-dimensional structure. Further, it relates to a novel polyimide precursor making it possible to produce the polyimide of the present invention and the thin film thereof by a simple method which is not likely to cause gelation, and a solution thereof.
The polyimide thin film of the present invention is used as a layer insulating film for integrated circuits (LSI) in microelectronics and an aligning film for liquid crystal displays.
2. Description of the Related Art
In recent years, fineness and high speed of LSI using copper wires are desired. A dielectric constant of a layer insulating film has to be reduced in order to meet them, and development thereof is actively carried out at present, which results in successive publication of the new materials. In the greater part thereof, a dielectric constant thereof is reduced by making use of a dielectric constant (1.0) of the air to introduce a hole into a material. However, simple dispersion of holes in the structure has brought about the problem that an increase in the hole rate results in a reduction in the mechanical strength.
In order to solve this problem, it is tried in several cases to allow a mechanical strength and a low dielectric constant to stand together by controlling a size of the holes and a form of the material at a nanometer level. For example, disclosed in Shingaku Technical Report, SDM2000-194 (2001) is the proposal that a limiting low dielectric constant and a high strength can be achieved by regularly forming nanoholes of a molecular level between structural units of a three-dimensional organic polymer having a pseudo-diamond structure.
Further, polyimide comprising tetraminoadamantane and benzenetetracarboxylic acid is disclosed in Japanese Patent Laid-Open No. 332543/2001, but polyimides having the other structures are not disclosed at all.
The most general method for producing a polyimide thin film is a method in which diamine and tetracarboxylic dianhydride are subjected to polycondensation reaction in a solvent to prepare a polyamic acid solution and in which it is then dehydrated by heating or a chemical method to form an imide ring. However, when the above method is applied to multifunctional amino compounds higher than triamine, a three-dimensional cross-linking structure is formed by polyaddition reaction, and a solvent is taken thereinto to cause gelation. A gel has the defects that it is an insoluble swollen substance and uneasy to handle and that it is very difficult to form a molded matter such as a thin film.
A vapor deposition polymerization method is introduced as a polyimide synthetic method using no solvent in Salon et al., Journal of Vacuum Science and Technology 1986, vol. A4, p. 369, but an expensive vapor deposition apparatus is needed for this vapor deposition method, and it is not necessarily a simple and industrial method.